1. Field of the Invention
The present invention relates to an atrial synchronous heart pacemaker, and in particular to a heart pacemaker switchable between a non-atrial synchronous and an atrial synchronous mode.
2. Description of the Prior Art
In heart pacemakers capable of switching between an atrial synchronous pacing mode and a non-atrial synchronous pacing mode, the generation of ventricular stimulation pulses in the atrial synchronous mode ensues after an AV delay time following the appearance of an atrium signal. Depending upon the selected smallest synchronous interval SSI, the ventricular stimulation frequency may vary within a relatively wide range from, for example, 50 through 150 impulses per minute. Such pacemakers generally contain a limit stage which determines the highest pulse rate. If an atrial signal occurs before a predetermined limit, which is derived from the difference between the smallest synchronous interval and the AV delay, the heart pacemaker switches to a non-atrial synchronous mode.
Although atrial synchronous heart pacemakers offer improved hemodynamics, it is known that a number of disadvantages exist in the use of such pacemakers. One such disadvantage is that a retrograde conduction in the heart may lead to a pacemaker-mediated tachycardia (PMT). In order to prevent PMT, it has been proposed to lengthen the refractory time in the atrium. Lengthening the refractory time, however, requires a reduction of the upper heartbeat frequency. Another proposal has been to extend the refractory time in the atrium only when the probability for a retrograde VA transition is high, such as, for example, following a premature ventricle contraction (PVC). As more recent investigations have shown, however, this approach can lead to a total inhibition of the heart pacemaker under certain circumstances, as reported in the periodical PACE, Vol. 8, November/December, 1985.
Whenever the heart pacemaker is to switch from the non-atrial synchronous mode e.g. VVI back to the atrial synchronous mode e.g. (DDD), the risk is particularly high that the synchronization will start precisely at an atrial activity triggered by retrograde transition, and thus will initiate a pacemaker-mediated tachycardia.